Detect current CMake version using CMake

There exist a few variables for that, described here:

CMAKE_MAJOR_VERSION
major version number for CMake, e.g. the “2” in CMake 2.4.3
CMAKE_MINOR_VERSION
minor version number for CMake, e.g. the “4” in CMake 2.4.3
CMAKE_PATCH_VERSION
patch version number for CMake, e.g. the “3” in CMake 2.4.3

Also, the variable CMAKE_VERSION contains the string for the version.
In your case, you would, for instance, use the following:

if(${CMAKE_VERSION} VERSION_LESS "3.8.0") 
    message("Please consider to switch to CMake 3.8.0")
endif()

Other comparison operators are VERSION_EQUAL and VERSION_GREATER.